Australochelys
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''Australochelys'' is an extinct
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of rhaptochelydian
turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
. It is known from one species, ''A. africanus'', that came from the
Elliot Formation The Elliot Formation is a geological formation and forms part of the Stormberg Group, the uppermost geological group that comprises the greater Karoo Supergroup. Outcrops of the Elliot Formation have been found in the northern Eastern Cape, south ...
of
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
. The holotype of ''Australochelys'' consists of only a
skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
and a fragment of the carapace, which shows both primitive and derived features. Like '' Proganochelys'', ''Australochelys'' has large orbits and a ventral basioccipital tubercle, but like derived turtles such as casichelydians, a group containing
Cryptodira The Cryptodira ('' el, hidden neck'') are a suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira (side-necked turtles) in that they lower their necks and pull the heads straight back into the ...
and
Pleurodira The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. The division between these two suborders represents a very deep evolutionary divide between two very different types of turtles. The physical differen ...
, it possesses a sutured basipterygoidal attachment, and a middle ear region partially enclosed laterally. These characteristics show that ''Australochelys'' is more closely related to casichelydians than to ''Proganochelys'', and together with the former, it makes up Rhaptochelydia. The skull of ''Australochelys'' shows that an advanced hearing mechanism of turtles evolved before the appearance of modern turtles.


Etymology

''Australochelys africanus'' was named in 1994 by
Eugene S. Gaffney Eugene S. Gaffney is an American paleontologist and leading authority on the morphology and evolutionary history of turtles. Biography Gaffney graduated from Rutgers University, Rutgers State University in 1965 and received his PhD in 1969 with a ...
and James W. Kitching. The generic name comes from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''australos'', meaning "south" and ''chelys'', "turtle". The specific name is derived from the Greek ''africanus'', meaning "from Africa".


Description


Skull

The holotype of ''Australochelys'' includes a
skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
and a fragment of the carapace. The skull shares features with both the primitive turtle '' Proganochelys'' and Casichelydia, a group of derived turtles. The skull shows an advanced hearing mechanism, a feature thought to have evolved with modern turtles.


Distinguishing characteristics

Many features were found among ''Australochelys'' that function to distinguish it from other turtles. Below are the characteristics found by Gaffney and Kitching in 1995 that are unique among all turtles: * an orbit larger in relative size than in any other turtle; * a lacrimal foramen at least three times larger than in ''Proganochelys''; * the external
nare A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called tur ...
s elongated in contrast to all turtles; * and
vomer The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right max ...
s arched dorsally, narrow posteriorly, and very broad anteriorly in a unique configuration. Below are features that are only found in turtles between ''Australochelys'' and ''Proganochelys'', but also present in tetrapods: * the presence of nares formed by the
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
divided external; * large interpterygoid vacuity like in ''Proganochelys''; * an unenclosed middle ear region; * a recessed, funnel-shaped, cavum tympani absent; * lacrimal foramen present; * recessus scalae tympani and fenestra perilymphatica absent; * foramen jugulare posterius with at least a medial edge absent; * and the presence of a cultriform process. Below are characteristics found in ''Australochelys'' and Casichelydia but not in ''Proganochelys'': * a sutured basipterygoid articulation; * a stapes that probably does not articulate directly with quadrate but may have attached to a tympanic membrane supported by the acute posterior edge of the quadrate; * a distal end of opisthotic covered by quadrate; * a well-defined canalis cavernosus canal; * few or no palatal teeth; * a distinct crista supraoccipitalis vertical bone sheet; * and a temporal roof that extends posteriorly over opisthotic.


Classification

''Australochelys'' shows a strange mix of basal and derived characteristics. This led Gaffney and Kitching to classify it in two new groups, Rhaptochelydia and Australochelyidae. Rhaptochelydia includes Australochelyidae and Casichelydia, a group including the derived groups
Cryptodira The Cryptodira ('' el, hidden neck'') are a suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira (side-necked turtles) in that they lower their necks and pull the heads straight back into the ...
and
Pleurodira The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. The division between these two suborders represents a very deep evolutionary divide between two very different types of turtles. The physical differen ...
. Below is a simple
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
found by Gaffney and Kitching in 1995 demonstrating the relationships of ''Australochelys'':


Paleoecology

''Australochelys'' is from the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...
Elliot Formation The Elliot Formation is a geological formation and forms part of the Stormberg Group, the uppermost geological group that comprises the greater Karoo Supergroup. Outcrops of the Elliot Formation have been found in the northern Eastern Cape, south ...
, a formation that dates around 190 to 180 million years ago. This makes it the oldest
africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
n turtle, a record that was previously held by a fragmentary specimen from the Gokwe Formation, which was dated to the
Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...
. Other than the fragmentary possible remains, the oldest definite specimens date to the Early Cretaceous of Africa, which shows about a 60 million year gap between themselves and ''Australochelys''. The holotype of ''Australochelys'' was found in a '' Tritylodon'' horizon, in the middle section of the formation.


References

{{Taxonbar, from=Q9162394 Jurassic reptiles of Africa Prehistoric turtle genera Taxa named by Eugene S. Gaffney Taxa named by James Kitching Testudinata